1. Field of the Invention
The present invention relates to a testing integrated circuit for measuring an output voltage of an analog circuit in an internal circuit of LSI.
2. Description of the Related Art
There are often cases in which a voltage of an internal circuit of LSI must be measured to evaluate the reliability of LSI. For example, the following methods for measuring the voltage of the internal circuit can be considered at the trial manufacture:
FIG. 1 shows a structural view explaining the voltage of the internal circuit of an LSI chip by a testing probe. At the time of the trial manufacture, the surface of the chip is exposed to facilitate the examination and analysis. A small pad 22 is formed on an LSI chip 21 to measure the voltage. The small pad 22 is formed such that a probe can contact thereto in advance. A probe 23 is brought into contact with the small pad 22. A signal is suitably supplied to a pad 24 and an output of an internal circuit 25 is measured by a voltmeter 26 connected to the probe 23. By use of the above-mentioned structure, it is possible to measure the voltage of the target portion at an early stage immediately after a wafer is manufactured.
However, in the above-mentioned measuring method, it is complicated to bring the probe 23 into contact with the small pad 22, and it takes much time to measure the voltage. Moreover, such measurement using the probe 23 cannot be performed after assembly such as packaging. Furthermore, in a case where the small pad 22 is not provided in advance, it becomes very difficult to contact the probe 23 to a wire itself.
At the time of the production of the LSI chip in which the trial manufacture is finished, there is a case in which the voltage of the internal circuit is measured and inspected to discriminate between good products and inferior products. This is a method in which a wire of the circuit to be measured is extended to a bonding pad and voltage is measured from the outside of the pad after assembly. Since this method can be performed during a general product inspection, required time for measurement is reduced. It is possible to perform such measurement before or after packaging. However, a pad for measurement must be provided. There is another cases however, in which a pad for measurement cannot be newly increased in view of the chip layout. In such a case, other pads can be used as a pad for measurement. However, in such a case, an extra burden must be imposed on the design.
In the above two examples, of course, a voltmeter for measuring a voltage must be provided.
Moreover, in recent years, a method had been widely used in which an electron beam is directly incident on the wire on the chip, and a signal is observed by the amount of mobile electrons. This method is excellent in resolution, but improper for voltage (current) measurement.